1. Field of the Invention
The present invention relates to wireless communications networks, and more specifically, to medium reservation in multiple channel environments.
2. Description of the Related Art
When multiple communications devices in a network share a common communications medium, some form of medium-reservation protocol is required to support that sharing. Medium-reservation protocols include carrier-sense multiple-access (CSMA), time-division multiple-access (TDMA), frequency-division multiple-access (FDMA), and code-division multiple-access (CDMA). Media include wired (e.g., electrical bus) and wireless (e.g., a band of electromagnetic spectrum) varieties.
CSMA is a medium access control (MAC) protocol in which a device verifies the absence of other traffic on a medium before transmitting on the medium. “Carrier-sense” describes the fact that a transmitter will first listen for a carrier on the medium before attempting to transmit on the medium. This is done in order to attempt to avoid collisions on the medium, such collisions reducing the ability of receivers to properly receive transmission on the medium. “Multiple-access” describes the fact that the protocol is designed to allow multiple devices to share the same medium.
In pure CSMA, a transmitting device does not detect collisions, and a receiving device does not distinguish between collisions and other sources of errors. Instead, if a receiving device properly receives a transmission, the receiver sends an acknowledgement (ACK) to the transmitter. If the transmitter receives no ACK, it backs off for a random period of time, performs carrier sense again, and reattempts the transmission after determining that the medium is no longer busy.
In an attempt to improve upon the performance of pure CSMA, modern networks typically employ one of two common variants of CSMA, namely CSMA with collision detection (CSMA/CD) or CSMA with collision avoidance (CSMA/CA).
Ethernet networks and networks conforming to the IEEE 802.3 standard, for example, are wired networks that employ CSMA/CD. Under CSMA/CD, a transmitting device has the ability to detect collisions on the medium by listening to its own transmissions on the medium and noting any errors that occur. In response to collision detection, a transmitting device will immediately stop transmission and back off for a random interval before again attempting to transmit. The IEEE 802.3 standard is described in more detail in “IEEE 802.3 Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications,” Institute of Electrical and Electronics Engineers (IEEE), 2002, incorporated herein by reference in its entirety.
Networks conforming to the IEEE 802.11 standard, on the other hand, are wireless networks that employ CSMA/CA. Under CSMA/CA, a device that intends to transmit a data frame will first monitor the medium and then transmit the data frame if the medium is free. If the medium is not free, the device will wait until the medium is free, and then back off for a random interval before attempting to transmit. In either case, the device will wait for an acknowledgement that its transmission was successful before proceeding to transmit another data frame. If no acknowledgement is received, the device will retry the transmission. IEEE 802.11 is described in more detail in IEEE Standard 802.11, 1999 (Reaff 2003) Edition, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications,” Institute of Electrical and Electronics Engineers (IEEE), 2002 (which includes IEEE Std 802.11, 1999 Edition; IEEE Std 802.11a-1999; IEEE Std 802.11b-1999; IEEE Std 802.11b-1999/Cor 1-2001; and IEEE Std 802.11d-2001) (herein “the IEEE 802.11 standard”) specifically, sections 7.2.1.1, 7.2.1.2, 9.2.5.4, and 9.2.5.6, incorporated herein in their entirety by reference.
Medium-request signals are an important part of medium reservation. However, in some multiple-channel networks, medium-request signals can create inefficiencies. For example, many present-day mass-produced wireless communications devices have receivers with limited frequency-band selectivity. Such receivers have a limited ability to reject signals from outside their tuned channel or intended frequency band of operation. These receivers will sometimes, therefore, pick up a medium-request signal from a neighboring channel, such neighboring-channel medium-request signal potentially causing an unnecessary pause in communication on the tuned channel.
For example, in a multiple-channel wireless network such as is specified by the IEEE 802.11 standard, a medium-request signal (e.g., a request-to-send (RTS) frame or a clear-to-send (CTS) frame) that is sent by a device on a first channel (e.g., channel 1), can potentially be received by devices on a second channel (e.g., channel 2). The receipt of an RTS frame by devices on the first channel properly reserves the medium of the first channel for transmission by the sending device on the first channel. Unfortunately, in this scenario, it also blocks communication between devices on the second channel. This blocking of transmissions on the second channel can cause a significant drop in transmission efficiency in a multiple-channel network. Furthermore, the IEEE 802.11 standard actually specifies channels that overlap in frequency. Use of these adjacent frequency-overlapping channels is not recommended within the same sub-region of a network; however, spatial overlap of specific channels often occurs on the boundaries between wireless network sub-regions, exacerbating the efficiency issues in those and surrounding areas, depending on the specific protocols in use.
Accordingly, there exists a need for medium-reservation protocols that improve transmission efficiency in multiple-channel environments.